A known breakage prediction method in the related art involves setting a cutting-tool breakage-risk torque level with a magnitude just prior to reaching breakage of a cutting tool from a cutting torque pattern obtained when a test workpiece is experimentally cut by using a new cutting tool, and comparing the set cutting-tool breakage-risk torque level with a cutting torque pattern detected when an actual workpiece is cut, so as to predict breakage of the cutting tool (for example, see Japanese Unexamined Patent Application, Publication No. Hei 6-198547).
In this breakage prediction method, the torque pattern is detected by disposing a torque sensor between a tool holder that supports the cutting tool and a motor that drives the tool.